FIG. 1 is a cross-sectional view of a conventional semiconductor package 1′. Package 1′ includes a semiconductor chip 2 that is attached to a planar upper surface 5a of a heat sink 5′ using adhesive 6. Heat sink 5′ has a relatively large thickness (e.g., 1 to 3 mm) and may be formed of copper, aluminum, or other materials. A plurality of metal leads 7′ surround semiconductor chip 2. Leads 7′ are about 0.12 mm to 0.15 mm thick, and thus are much thinner than heat sink 5′. Each lead 7′ comprises an encapsulated inner lead 11′ and a nonencapsulated outer lead 12′. The inner leads 11′ overlap heat sink 5′, but are electrically isolated from heat sink 5′ by a dielectric material, e.g., a layer of an adhesive tape 6a. Metal bond wires 3 electrically connect each inner lead 11′ to a bond pad of semiconductor chip 2. An insulative, molded resin encapsulate 4 forms the package body and covers semiconductor chip 2, inner leads 11′, conductive metal bond wires 3, and upper surface 5a′ and side surfaces 5b′ of heat sink 5′. Planar lower surface 5c′ of heat sink 5′ is exposed at the lower surface of the resin encapsulate 4 in order to obtain improved heat discharge characteristics.
FIG. 2 illustrates a conventional procedure for fabricating a conventional heat sink 5′. In particular, a pair of facing U-shaped slots 51′ are stamped through a metal sheet 50′. A pair of opposing support bars 52′ remain after slots 51′ are stamped. Subsequently, support bars 52′ are cut in a second stamping step. Since the support bars 52′ are relatively thick, some elongation of support bars 52′ occurs during the cutting operation. As a result, V-shaped protrusions 8 are formed on two opposing sides of heat sink 5′. This two step stamping process is used because of the substantial thickness of metal sheet 50′ and support bars 52′. If a single stamping step were used instead, heat sink 5′ would be bent. Consequently, an additional step to flatten heat sink 5′ would be required.
After heat sink 5′ is cut from metal sheet 50′, heat sink 5′ typically is subjected to several complicated coating and treatment steps. For example, the exposed lower surface 5c′ of heat sink 5′ (FIG. 1) typically is sand blasted to facilitate marking and then plated with nickel. The encapsulated surfaces of a copper heat sink 5′ are subjected to a well-known black oxidation process (adapted to form a CuO thin film and/or a Cu2O thin film) that facilitates the attachment of encapsulate 4 to heat sink 5′.
Conventional heat sink 5′ of package 1′ and the methods used to make heat sink 5′ have several disadvantages. First, as described above, heat sink 5′ is too thick to be stamped out in a single stamping step, but rather requires two stamping steps. Second, because lower surface 5c′ of heat sink 5′ is exposed at the lower surface of the package body, the complicated nickel coating and sand blasting steps described above are necessary. Third, protrusions 8 on heat sink 5′ cause turbulence in the flow of resin during the molding process, and possibly can cause the formation of undesirable voids in encapsulate 4. Fourth, because heat sink 5′ is heavy, and is much thicker than inner leads 11′, inner leads 11′ may become bent during handling of the leadframe after heat sink 5′ is attached thereto. Such a bend in the leads may cause short circuiting and may adversely affect wire bonding. Fifth, because lower surface 5c′ of heat sink 5′ is exposed, a more complicated mold is required than would be used for an ordinary leadframe that does not have an exposed heat sink. Finally, excess encapsulate flashes onto lower surface 5c′ of heat sink 5′ during molding. Accordingly, a deflash step is necessary to remove the excess molding compound. This deflash process typically includes a chemical deflash step, followed by a mechanical deflash step using a water jet rinse.